The high-pressure sodium lamp has been commercially available for many years. Because of the color rendering properties of the standard sodium vapor lamp, their use has been generally restricted to roadway, industrial and similar floodlighting applications where color rendition is not of particular importance. Because of the high-pressure sodium lamp's relatively poor color rendering properties it has not found its way into commercial, school, retail or other color-sensitive applications. These commercially available high-pressure sodium discharge lamps have a color rendering index (CRI) of generally between 15 and 25, but perform with an efficacy of between 100 and 110 lumens per watt.
In designing lamps for optimum efficiency it has been necessary to maintain the sodium pressure on the order of about 100 torr at the operating temperature and the reversal of the sodium D line should be within a fairly wide range of between about 4 nm and 10 nm depending upon the amalgam ratio. The relationship between the efficiency and the sodium D line reversal is a well-know phenomenon. It is also well known that the CRI of the high-pressure sodium lamp improves as the sodium pressure in the lamp increases, as was pointed out in the paper "New High-Pressure Sodium Lamp With Higher Color Rendition" by H. Mizuno, et al. CIE XVII Session, Barcelone 1971. For example, if the currently marketed lamps are overloaded to run at higher wattages, the color rendering properties of the lamp are significantly improved. This improvement is achieved at considerable expense of a shortened lamp life.
It has also been known that increasing the sodium vapor pressure by means of increased temperatures will improve the lamp color, but operating these lamps at the temperatures required have presented problems with seal failure and results in extremely short lamp life. It has also been known for many years and disclosed in U.S. Pat. Nos. 3,723,784; 3,974,410 and 4,001,634 to employ metal bands or shields about the ends of the arc tube in order to increase the temperature in the area of the coldest part of the arc tube.
Producing a high-pressure sodium vapor discharge lamp with a CRI value of between 65 and 75 without a significant loss in lamp efficacy would provide a source of illumination which is both energy-efficient, has a reasonably long life and would be suitable for applications such as schools, offices and retail operations where proper color rendition is of sigificant importance.